The strategy for implementing damage detection and the characterization of mechanical structures is commonly called Structural Health Monitoring (SHM). Damages are defined as modifications of the material and/or of the geometrical properties of a structural system, comprising modifications of boundary conditions and connections of the system, that worsen performances of the system. The SHM process implies the observation of the mechanical system over the time using periodically: measurements of dynamic responses coming from an array of sensors, extraction of data of damage characteristics sensed from these measurements, and statistical analysis of these data of characteristics for determining the present health state of the system (also called structural analysis).
The (periodically updated) output of this process is an information about the capacitance of the structure of carrying out its function, considering the unavoidable aging and of degradation in working environments. After extreme events, such as earthquakes or explosions, the SHM is used for a quick screening of the conditions of the structure for providing, almost in real time, reliable information about the integrity of the structure itself.
Nowadays, SHM systems use sensors located outside the surfaces to be controlled. For example, in bridges a number of sensors are used (anemometers for calculating the wind speed, accelerometers, extensometers, motion transducers, temperature sensors, sensors for detecting motion of weights, etc.), which are placed on the external surfaces of beams, ropes or pillars, in order to estimate the effects of loads on the bridge, evaluate the weakening of the bridge, and foresee the probable evolution of the bridge and its expected lifetime.
There are technical reasons that hinder the realization of cost efficient SHM with sensors buried in the same structures to be monitored. In particular, any sensor (of pressure, moisture, temperature, etc.) inside a block of reinforced concrete should be connected to an antenna or to a conductor for communicating outside the block itself the sensed measurements.
Let us consider for example a sensor, buried in a block of concrete, that should communicate to the outside through an antenna buried in the same block. The block of reinforced concrete may cause an attenuation that could range between 0.5 and 1 dB per cm of thickness. See Xiaohua Jin, M. Ali “Reflection and Transmission Properties of Embedded Dipoles and PIFAs inside Concrete at 915 MHz” Antennas and Propagation Society International Symposium, 2009, Print ISBN: 978-1-4244-3647-7. This imposes, at least, a maximum communication distance between the buried sensor (supposedly not supplied by a local battery) and an external sensor, fixed by an energy budget.
Moreover, electromagnetic characteristics of reinforced concrete (different from those of air) cause a reduction of the resonance frequency of the antenna (antenna detuning), that should be taken into consideration while designing. Finally, the presence of metal structures close to the antenna causes interferences that could jeopardize reliability of communications towards the outside.
If a battery connected to the sensor was used instead, there would be time limits of use of the sensor fixed by the battery charge.
The antenna and the radio part of present SHM sensors are physically connected to each other through bond-wire (or bump). This causes reliability problems because inside blocks of pre-compressed reinforced concrete in steady-state conditions (attained after about 1 month), there are pressures of the order of hundreds of atmospheres that make this physical connection between the antenna and the radio part of the SHM sensor very difficult. The poor reliability of this physical connection between antenna and radio part of the SHM sensor is the reason that explains why no sensor is simply connected to the outside through an electric wire (wired connection).